systemtap: Unbreak build for systems with nss headers installed

systemtap: Unbreak build for systems with nss headers installed

* discussed on [1]
* patch sent to systemtap [2]

[1] http://lists.linuxtogo.org/pipermail/openembedded-core/2012-June/023377.html
[2] http://sourceware.org/ml/systemtap/2012-q2/msg00202.html

(From OE-Core rev: bd17907884e3a1e1d711125a4b8def5263e97edb)

Signed-off-by: Andreas Müller <schnitzeltony@googlemail.com>
Signed-off-by: Elizabeth Flanagan <elizabeth.flanagan@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>